Under a deal signed last month by Kerry and Lavrov in Geneva a truce was declared on Sep 12 but it collapsed within a week.

Washington accused Moscow of failing to rein in Assad's government forces and of itself carrying out air strikes on civilian targets, including a UN aid convoy.

Moscow, meanwhile, says the United States failed to separate "moderate" anti-Assad rebels from militants linked to Al-Qaeda.

According to Kirby, Russia was "unwilling or unable to ensure Syrian regime adherence to the arrangements to which Moscow agreed."

The US spokesman accused Moscow and Damascus of targeting hospitals and preventing aid from reaching desperate civilians.

The Russian response was just as stark. "Washington simply did not fulfil the key condition of the agreement," Zakharova said of Washington's struggle to separate opposition fighters from the militant Fateh al-Sham Front.

"After failing to fulfill the agreements that they themselves worked out, they are trying to shift responsibility on to someone else."

Meanwhile, the fighting continued. The Pentagon confirmed reports that US warplanes had targeted a "prominent" Al-Qaeda leader near Idlib in northern Syria.

This came as the Fateh al-Sham Front, which recently changed its name from Al-Nusra Front following a break with Al-Qaeda, confirmed the death of veteran commander Ahmed Salama Mabrouk.

'SPEECHLESS SHOCK'

The aid agency Doctors Without Borders reacted with fury to the latest hospital bombings, saying strikes on clinics in Syria and Yemen leave "the global medical community in speechless shock."

This came after an air raid destroyed rebel-held eastern Aleppo's biggest hospital, the M10.

"The hospital is now not usable at all," said Adham Sahloul of the Syrian American Medical Society. "It is not salvageable, per reports from the staff and doctors there." He said three maintenance workers were killed.

Moscow on Monday denied it was bombing hospitals, insisting its bombing campaign was "highly effective" and had stopped militants taking over Syria.

Elsewhere, at least 21 Turkish-backed Syrian rebels fighting the Islamic State group were killed on Sunday by landmines laid by the militant group, the Syrian Observatory for Human Rights said.

The toll was the highest loss of life for forces involved in the Turkish-led operation in northern Syria since it began in late August, it said. It did not mention any Turkish casualties.